In recent years, methods of producing a sugar liquid by pretreating cellulosic biomass with an acid, hot water, alkali or the like and then adding cellulase thereto to perform hydrolysis have been widely studied. The thus obtained sugar liquid sometimes has a lower sugar concentration than a conventional sugar liquid derived from an edible material such as starch or cane juice.
In general, when the sugar concentration is low, the sugar concentration can be increased by distilling water in the sugar liquid by, for example, concentration under reduced pressure or concentration by heating. For example, when beet molasses is concentrated in an evaporator has been disclosed (see JP 2005-278407 A).
On the other hand, in methane fermentation treatment of organic wastes, there have been troubles caused by precipitation of alkaline earth metals including calcium and magnesium contained in the wastes such as clogging of pipes and deterioration of functions of separation membranes due to their attachment on the membrane surfaces (see WO 2009/041009).
We found problems in culture of microorganisms using as a fermentation feedstock a cellulosic biomass sugar liquid, especially concentrated cellulosic biomass sugar liquid, that precipitation of an insoluble substance containing magnesium as a major component may cause attachment of scale to a fermentation apparatus, clogging of pipes, clogging of a separation membrane, occurrence of a trouble in a pH/DO sensor, attachment of scale to a separation membrane during continuous culture, and difficulty in membrane separation of a fermentation product from the culture liquid.
It could therefore be helpful to provide a method of producing a sugar liquid, which method can prevent the above problems, that is, attachment of scale to a fermentation apparatus, clogging of pipes, clogging of a separation membrane, occurrence of a trouble in a pH/DO sensor and attachment of scale to a separation membrane during continuous culture, and enables membrane separation of a fermentation product from the culture liquid.